According to one embodiment, a surface light source device includes a light guide plate, and first and second light-emitting units. The light guide plate includes a flat portion and a curved portion. The curved portion guides, to the flat portion, light introduced from the end surface and transmits light introduced from the inner circumferential surface. The flat portion includes a light diffusing portion to diffuse the light guided by the curved portion, and a light-emitting surface to output the light diffused by the light diffusing portion. The first light-emitting unit emits light toward the end surface. The second light-emitting unit emits light toward the inner circumferential surface.